This invention relates to a color xerographic printer and, more particularly, to a color xerographic printer with a monolithic structure of multiple linear arrays of surface emitting lasers with dissimilar polarization states and dissimilar wavelengths to simultaneously expose widely separated positions on the same or different photoreceptors.
A Raster Output Scanner (ROS) or a Light Emitting Diode (LED) print bar, known as imagers, used in xerographic printers are well known in the art. The ROS or the LED print bar is positioned in an optical scan system to write an image on the surface of a moving photoreceptor belt.
In a ROS system, a modulated beam is directed onto the facets of a rotating polygon mirror which then sweeps the reflected beam across the photoreceptor surface. Each sweep exposes a raster line to a linear segment of a video signal image.
However, the use of a rotating polygon mirror presents several inherent problems. Bow and wobble of the beam scanning across the photoreceptor surface result from imperfections in the mirror or even slight misangling of the mirror or from the instability of the rotation of the polygon mirror. These problems typically require complex, precise and expensive optical elements between the light source and the rotating polygon mirror and between the rotating polygon mirror and the photoreceptor surface. Additionally, optically complex elements are also needed to compensate for refractive index dispersion that causes changes in the focal length of the imaging optics of the ROS.
The LED print bar generally consists of a linear array of light emitting diodes. Each LED in the linear array is used to expose a corresponding area on a moving photoreceptor in response to the video data information applied to the drive circuits of the print bars. The photoreceptor is advanced in the process direction to provide a desired image by the formation of sequential scan lines.
In a color xerographic printer, a plurality of the light emitting elements of the LED print bars are imaged to a photoreceptor surface usually by closely spaced radially indexed glass fibers known as "selfoc" lenses.
Printing with LED bars requires a precisely fabricated "selfoc" lens for each light emitting element. Each "selfoc" lens array must be straight and parallel with highly polished input and output facets. Each lens within the array must have the same focal length and throughput efficiency. Even if these requirements are met, the "selfoc" lenses have short focal lengths and therefore must be positioned close to the photoreceptor surface where the lenses can collect toner and thereby require an additional cleaning mechanism. Due to their optical characteristics, the depth of focus of a "selfoc" lens is very short and consequently requires very precise placement to produce uniform spot exposures on the scan line.
Light emitting diodes, by their very nature, have a large angular divergence, a broad spectrum and are unpolarized, all factors which severely limit their use in color printing systems using a wavelength or polarization based scan line separation technique. Prior LED print bar xerographic line printers have taught only line exposure at a single position on one photoreceptor.
U.S. Pat. No. 5,337,074, commonly assigned as the present application and herein incorporated by reference, and U.S. Pat. No. 5,461,413 teach using a single linear surface emitting laser array as the light source for a line printer.
A laser array has a smaller angular beam divergence than an LED array and therefore provides a higher power throughput efficiency. A laser array also has a smaller radiating aperture (source size) than an LED array and therefore can provide increased spot density. The narrow spectrum of laser beams enables optical separation of the laser beams as taught in the present application. The broad spectrum precludes similar separations of LED emissions.
It is an object of this invention to provide a color xerographic line printer with simple and inexpensive optics and a single light source.
It is yet another object of this invention to provide a color xerographic printer with a multiple laser array light source with dissimilar wavelengths and dissimilar polarization states.